This research involves studies of how bacterial IgA proteases influence the function of secretory immunity in human beings. IgA proteases are extracellular endopeptidases which cleave only human immunoglobulin A of the IgA1 subclass to yield Fab and Fc fragments. A single prolyl-threonyl heavy chain bond is cleaved and the IgA2 subclass of human IgA, lacking the susceptible bond, is enzyme resistant. The enzymes are elaborated by Streptococci Lancefield Group H and Neisseria gonorrheae and N. menigitidis. Enzymatic cleavage of IgA reduces its effective antibody activity, suggesting that the enzymes may be immunosuppressive. Owing to its extreme specificity, IgA protease assay is difficult and requires human IgA (M.W. 170,000 Daltons) to be used as substrate. The enzyme activity is metal dependent; the pH optimum of activity is at 7.0 but a wide activity range is observed. BIBLIOGRAPHIC REFERENCES: Doellgast, GJ and AG Plaut. Purification of Human IgA by Salt-Mediated Hydrophobic Chromatography. Immunochemistry Volume 13: pp 135-139, 1976. Wistar, R., Jr., Plaut Ag and J.V. Gilbert. Susceptibility of Human IgA Antibodies to Microbial IgA Proteases. Clin Res. 24, 454a 1976.